An electromagnetic filter provides frequency-dependent attenuation of electromagnetic signals propagating through a circuit. A bandpass filter selectively permits signals of frequencies within a predetermined passband to pass with minimal loss, while a stopband filter, also referred to as a notch or band-reject filter, suppresses signals of frequencies within a predefined rejection band. A variety of frequency-dependent attenuation profiles are obtainable by combining the properties of band-reject and bandpass filters. Filters can be further categorized as passive or active, and fixed- or variable-tuned.
Fundamental to filter configurations is a resonator designed to resonate, or "ring" at a prescribed resonance frequency. In well-known multipole filters, for example, the impedance and admittance poles of the filter are conferred by a multiplicity of resonators suitably coupled to one another and to the associated circuit. The resonator may be of the "lumped-element" type, composed of an inductor L and a capacitor C, a combination which is well known to possess the resonance frequency .function..sub.o =1/(2.pi..sqroot.LC) at which it spontaneously oscillates if excited, for example by means of an initial electric charge stored in the capacitor. If stimulated by means of an externally applied AC signal of frequency .function.,the resonator exhibits a more or less sharply defined peak in impedance (if L and C are connected in parallel) or admittance (L and C in series) in the frequency range centered at .function.=.function..sub.o. Or, the resonator may be of the transmission-line type, comprising a segment of transmission line relatively isolated from its associated circuit. In a well-known typical embodiment, the length of the segment is an integer multiple of one-half wavelength at the desired resonance frequency .function..sub.o. In an alternative embodiment, namely a transmission line in the form of a closed loop or ring, resonance occurs when the length is an integer multiple of one wavelength. Transmission-line and lumped-element resonators respond to electrical stimulation in precisely analogous fashion in the vicinity of their respective resonance frequencies; the principal difference in performance between the two is in that the transmission-line resonator exhibits a succession, or spectrum, of harmonic, or overtone resonance frequencies occurring when the length of the resonator equals an integer number of half-wavelengths. The excitation of a transmission-line resonator may be visualized as a propagating wave undergoing repeated internal reflections as it collides with the discontinuities at opposite ends of the transmission-line segment, or as a propagating wave closing in phase on itself in the ring resonator embodiment. In this respect, the resonance is analogous to that observed in musical instruments such as organ pipes and violin strings.
A filter whose passband or stopband is tunable by means of an electric control circuit has been the subject of active consideration for a variety of microwave systems, including radars and wireless telecommunication systems. To confer tunability, materials whose electromagnetic properties can be varied, such as ferroelectrics and ferrimagnetics, have been investigated for use as substrates on which planar-circuit resonator patterns are applied, thus providing means to control the effective propagation length, hence to vary the resonance frequencies. The method of present concern depends on the use of ferrimagnetic substrate materials whose permeability is controlled by application of a magnetic field. Examples include U.S. Statutory Invention Registration No. H432, and U.S. Pat. Nos. 5,426,402 and 5,448,211 to Mariani, directed to tunable band-rejection filters formed on dielectric/magnetic substrates. In each example, resonant slotlines are provided on a metallic surface proximal to a magnetized ferrite substrate. The permeability of the ferrite substrate changes as a function of the intensity of an applied magnetic field. This in turn changes the effective electromagnetic path length of the resonant slots and accordingly shifts the resonance frequency of the filter. Alternative control methods include use of ferroelectric materials whose permittivity can be electrically varied as described in Beall, J. A. et al, "Tunable High-Temperature Superconductor Microstrip Resonators", Digest of IEEE MTT-S International Microwave Symposium (1993), incorporated herein by reference.
The above example of prior art magnetically tunable filters and others generally require a high magnetic field to drive the substrate into a state of magnetic saturation and further to a condition such that magnetic resonance effects dominate the variation of permeability. This requirement imposes several disadvantages, including inconveniently large, heavy, and intricate magnet structures as well as limited speed and range of tuning. Furthermore, the strong magnetic fields in the prior art embodiments are generally oriented normal to the substrate, which gives rise to at least two disadvantages: incompatibility with superconducting performance; and the presence of a strong demagnetizing effect, therefore requiring a strong external field for operation. For these reasons, magnetically tunable filters have not lent themselves to the evolving technology of microwave planar circuits, in which minimization of size, weight, cost, and dissipative energy loss, and maximization of tuning or switching speeds are usually essential.